Since the morning the guards were discovered and debriefed, the FBI and private detectives hired by the museum have tracked hundreds of leads and dealt with dozens of intermediaries for individuals who contend they can lead investigators to the missing artwork. Invariably, the trails have come to dead ends, as information could not be corroborated or tipsters proved to be fakers, with an eye only for the reward money, said Geoffrey J. Kelly, the lead FBI agent on the case.

But in late April 1994, the museum received an overture that Gardner officials regard as the most promising lead ever in the case. An anonymous letter writer said he could facilitate the return of the paintings in exchange for $2.6 million and full immunity from prosecution for the thieves and those who held the paintings. Because the overture involved a request for immunity from prosecution, the museum turned the letter, postmarked in New York, over to the FBI.

The letter writer showed considerable knowledge of the paintings and of the international art world. He said the stolen paintings were being stored in archival conditions, and had not yet been sold. But, he warned, the museum should agree swiftly to the exchange because the paintings were being held in a country where a buyer who did not know they had been stolen could claim legal ownership.

Boston Globe Archive

On May 1, 1994, the number “1” was listed a few spaces in front of the actual US dollar exchange rate for the lira. This was intended to send a message to a possible Gardner heist suspect.

The writer proposed a clandestine way for the museum to respond to the overture. If the Gardner was open to negotiating a ransom deal, it should send a signal to him by arranging to have the numeral ''1" inserted in the US-foreign dollar exchange listing for the Italian lira that would be published in The Boston Sunday Globe on May 1, 1994. And, in fact, that Sunday, the numeral ''1" was listed a few spaces in front of the actual US dollar exchange rate for the lira.

Matthew V. Storin, editor of The Globe in 1994, said he was told of the letter's contents and agreed to insert the numeral -- being careful not to make the currency listing itself inaccurate -- at the request of Richard S. Swensen, the special agent in charge of the FBI Boston office.

''I saw it as a community-service decision," Storin said, adding that he cleared the move with William O. Taylor, the Globe's publisher at the time, and made it clear to Swensen that he expected the paper to get the first word if the overture led to the paintings' return.

The following week, the museum received a second letter from the anonymous writer. He was encouraged to see that the museum was interested in negotiating an exchange. But he was alarmed by the aggressive reaction by federal, state, and local law enforcement after the museum received his letter. Were the museum and authorities interested in getting the paintings returned, or in arresting a low-level intermediary, he wondered.

''YOU CANNOT HAVE BOTH," he wrote in capital letters, adding, ''Right now I need time to both think and start the process to insure confidentiality of the exchange."

If he decided it was impossible to continue negotiating, he wrote, he would provide the museum with some clues to the paintings' whereabouts. But he never wrote the museum again.

In the 10 years since the letter was written, federal prosecutors have dropped their opposition to giving immunity to someone who might want to return the artwork. US Attorney Michael J. Sullivan said he would be willing to forgo charges against anyone who facilitated the paintings' safe return. As for the actual thieves, the statute of limitations for prosecution ran out in the mid-1990s.